Currently, much effort is being devoted to the development of multichip electronic modules. Such modules, also referred to by some as "Hybrid Integrated Circuits" (HICs), are typically comprised of a substrate, usually ceramic, which has one or more layers, each layer having a pattern of metallization thereon, usually gold or the like. Very Large Scale Integrated (VLSI) circuit chips are usually bonded to one or both of the major (i.e., outer) surfaces of the substrate, with each chip interconnected to another by the metallized pattern(s) on the substrate. Selected features of the metallized patterns on each layer of the substrate are connected to features on adjacent layers by through-plated metal vias.
To reduce the overall size of such multi-chip modules, as well as to allow for very dense circuits, the lateral dimensions of the pattern features (e.g., their linewidth) are made very small. On average, the linewidth of the features ranges from 2 to 10 mils, depending on the nature of the process employed to deposit the metallization on the substrate. Maintaining precise control of the dimensions of the features in each pattern is very important. If the dimensions of an individual feature in a pattern exceed the maximum allowable value under the operative design rule, the likelihood of leakage of a signal between such a feature and one adjacent to it becomes much higher, leading to possible "cross-talk." Conversely, if the lateral dimensions of a particular feature within a pattern are too small, then the impedance of the signal path provided by that feature may exceed permissible limits, adversely affecting the operation of the multi-chip module. Obviously, a break in a feature within a pattern is also undesirable. Further, the quality of such features as vias is also important.
Not only are the lateral dimensions of the features in each pattern on the substrate important, but the overall quality of the metallization in the pattern is also important. During the process of depositing the pattern on each layer of the substrate, it is possible for the metallization in the pattern to become contaminated. Such contamination not only adversely affects the impedance of the circuit paths established by the pattern, but may also adversely affect the ability to reliably bond an integrated circuit chip to the metallization on the exposed surfaces of the substrate. Another critical parameter is the uniformity of the height of the features in the pattern. Significant variations in the feature height can adversely affect the quality of the electrical connections made to devices placed on the substrate.
In the past, inspection of a metallized pattern on a substrate to detect undesirable lateral dimensional variations has been performed by comparing the image of the pattern (as captured by a television camera) to the image of each of a pair of master patterns or models. Such models are typically generated so each contains an exact replica of the desired pattern of features, except that the lateral dimensions of the features of one have been eroded (shrunk) while the lateral dimensions of the features of the other have been dilated (expanded) to represent the minimum and maximum allowable values, respectively. By comparing the image of the actual pattern of metallization to each of the two models and then logically combining the results of such comparison, those features whose dimensions are too large or too small can be detected.
While techniques, such as the one described above, have been devised for accomplishing automatic pattern inspection to detect deviations in the lateral dimensions of the features, no comparable methods have been devised for accomplishing automated inspection of both the lateral dimensions, as well as one or more other important aspects of the features, such as their height, hue, intensity, reflectance, texture, saturation or any combination thereof. If there is any inspection of any of these aspects, such inspection is carried out manually. For example, inspection of the intensity of the features to determine the pattern's hue and coloration is usually carried out manually and is subjective at best. Moreover, the results may be adversely affected by operator fatigue.
Thus, there is a need for a technique for accomplishing automated inspection of a pattern of metallization on a substrate to detect not only lateral dimensional variations, but variations associated with at least one other aspect of the feature.